A mass spectrometry (MS) system in general includes an ion source for ionizing molecules of a sample of interest, followed by one or more ion processing devices providing various functions, followed by a mass analyzer for separating ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), followed by an ion detector at which the mass-sorted ions arrive. An MS analysis produces a mass spectrum, which is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. Mass spectrometers are commonly used to determine the chemical composition of mixtures by precise measurement of the mass-to-charge ratio of the constituent molecular ions.
An ion guide is an example of an ion processing device that is often positioned in the process flow between the ion source and the mass analyzer. An ion guide may serve to transport ions through one or more pressure-reducing stages that successively lower the gas pressure down to the very low operating pressure (high vacuum) of the analyzer portion of the system. For this purpose, the ion guide includes multiple electrodes that receive power from a radio frequency (RF) power source. The ion guide electrodes are arranged so as to bound an interior (volume) that extends along a central axis from an ion entrance to an ion exit, and has a cross-section in the plane transverse to the axis. The ion guide electrodes are further arranged so as to generate an RF electric field that limits the excursions of the ions in radial directions (in the transverse plane). By this configuration, the ions are focused as an ion beam along the central axis of the ion guide and are transported through the ion guide with minimal loss of ions.
The interior of an ion guide may be filled with a gas such that the ion guide operates at a relatively high (yet still sub-atmospheric) pressure. For example, a gas filled ion guide may be positioned just downstream of the ion source to collect the as-produced ions with as few ion losses as possible. Also, a buffer gas may be introduced into an ion guide under conditions intended to thermalize (reduce the kinetic energy of) the ions, or to fragment the ions by collision induced dissociation (CID). Some ion guides are structured so as to define a converging ion guide volume. The RF field applied by the converging geometry can compress the ion beam and increase the efficiency of ion transmission through the ion guide exit. The large beam acceptance provided by the entrance of the converging ion guide can improve ion capture, and the comparatively small beam emittance at the exit can improve ion transfer into a succeeding device and can be closely matched to the size of the inlet of the succeeding device. The converging ion guide can also operate more effectively at higher pressures than a non-converging ion guide.
One particular type of mass spectrometer is a time-of-flight mass spectrometer (TOF-MS), which utilizes a high-resolution mass analyzer (TOF analyzer) in the form of an electric field-free flight tube. An ion accelerator (or pulser) injects ions in pulses (or packets) into the flight tube. Ions of differing masses travel at different velocities through the flight tube and thus separate (spread out) according to their differing masses before arriving at the ion detector, enabling mass resolution based on time-of-flight. In a typical TOF-MS, ions travel along a drift direction through one or more gas-filled ion guides, and one or more beam-limiting apertures operating in a collision-free environment, and into the pulsed ion accelerator. In an orthogonal acceleration TOF-MS (oaTOF-MS), the ion accelerator receives the ions along the drift direction and injects the ions along an acceleration direction orthogonal to the drift direction.
Most instrument performance specifications are improved by increasing the ion transmission efficiency between the ion source and the ion detector. In absolute terms the net transmission efficiency is poor, in practice typically ranging from one part in 104 to one part in 109.
In the case of a TOF-MS, the inherent sources of ion loss include poor transmission through the beam-limiting apertures, the losses associated with the duty-cycle of the pulsed ion accelerator, and losses between the pulsed ion accelerator and the ion detector that are caused by the angular divergence of the ions in the TOF region (flight tube). Previous efforts have been made to improve each of these individual inefficiencies, but not necessarily simultaneously and not necessarily to the degree to which the net transmission from the gas-filled ion guide to the ion detector approaches 100% across a wide mass range. Additionally, previous efforts do not necessarily address the ion losses in high resolution TOF instruments, which have small apertures and long flight times. Lastly, known TOF-MS systems offer limited capability to mitigate the problems caused by ion-ion Coulomb repulsion in the ion guides.
Therefore, it would be desirable to minimize ion losses through all components of a TOF-MS system (or other MS system) between the ion guide and ion detector across a wide range of masses and intensities. It would also be desirable to minimize ion losses in a way that scales to high resolution TOF instruments.